The current invention is generally related to a two-way communication system between a mobile unit and a stationary unit, and more particularly related to a two-way communication system between a car and information stations located along a road.
A road communication system enables a ground vehicle such as a car to communicate with a traffic control or a road administration for disseminating or collecting information on traffic or road conditions. The information is gathered by stationary units along the road and is also collected by various sensors on a car. The information exchange is designed to help drivers avoid accidents as well as to automatically navigate the car without human intervention. One example of the above described road communication system is disclosed in Japanese Patent Laid Publication Hei8-241495. To implement the road communication system, it is necessary to place a continuous array of communication cells or stationary units along the roads. Although a leaking coaxial cable may be placed along the roads, not only the construction is significant, but also the radio signal travels only a short distance in a cross direction due to the low position of the cable. In contrast to the disadvantage of the cable, antennas are placed at a predetermined interval along the roads to communicate with cars. Each antenna covers a predetermined relatively wide transmission area or cell. Each antenna may be connected to a base unit of a road administrator via optical fibers.
In the prior art, each cell used a different frequency to prevent the communication from experiencing inter-carrier interference and or inter-symbol interference. Although the above system is advantageous for communication between stationary units, the above system may not be advantageous for the road communication system. Since the road communication system includes a mobile unit such as one used in a car which moves at a high speed and passes a number of cells over a short period time, it is necessary for a car to be equipped with a device for switching reception frequencies at a high speed. Consequently, a mobile unit needs a high-speed oscillator or a plurality of oscillators. The required additional hardware is either costly or takes physical space. For the above reasons, it was desired that a common frequency is used for the road communication system provided that the occurrence of the inter-carrier and inter-symbol interference is prevented. Japanese Patent Laid Publication Hei 9-358581 discloses ways to provide a stable signal containing the same data while a car moves across cells. In other words, the identical information is transmitted from every stationary unit. Some parts of Japanese Patent Laid Publication Hei 11-266194 are prior art in view of disclosures in its parent case Hei 9-358581 while other parts of Hei 11-266194 are not prior art.
The above prior art lacks disclosures on techniques for seamlessly communicating different sets of information between a stationary unit and a mobile unit while the mobile unit is moving across the cell boundaries of the stationary units. Furthermore, it has been desired that a common frequency is used for the road communication system to seamlessly communicate different data substantially without the carrier and code interference.
In order to solve the above and other problems, according to a first aspect of the current invention, a method of transmitting information via a common frequency carrier between a mobile unit and a set of stationary units, including: transmitting first data on a predetermined first subcarrier and second data on a predetermined second subcarrier between a first stationary unit and the mobile unit for communication within a predetermined first transmission area; transmitting third data on the predetermined first subcarrier and the second data on the predetermined second subcarrier between a second stationary unit and the mobile unit for communication within a predetermined second transmission area, the second stationary unit being located near the first stationary unit, the predetermined first transmission area and the predetermined second transmission area being partially overlapping; and maintaining communication among the mobile unit, the first stationary unit and the second stationary unit while the mobile unit is moving from the predetermined first transmission area to the predetermined second transmission area.
According to a second aspect of the current invention, a method of transmitting information via a common frequency carrier between a mobile unit and a set of stationary units, including: transmitting an information signal which is modulated by orthogonal frequency division multiplex modulation between a first stationary unit and the mobile unit for communication within a predetermined first transmission area; transmitting the information signal which is modulated by the orthogonal frequency division multiplex modulation between a second stationary unit and the mobile unit for communication within a predetermined second transmission area, the second stationary unit being located near the first stationary unit, the predetermined first transmission area and the predetermined second transmission area being partially overlapping; and maintaining communication among the mobile unit, the first stationary unit and the second stationary unit while the mobile unit is moving from the predetermined first transmission area to the predetermined transmission second area.
According to a third aspect of the current invention, a system for transmitting information via a common frequency carrier between a mobile unit and a set of stationary units, including: a first stationary unit for transmitting first data on a predetermined first subcarrier and second data on a predetermined second subcarrier within a predetermined first transmission area; a second stationary unit located near the first stationary unit for transmitting third data on the predetermined first subcarrier and the second data on the predetermined second subcarrier within a predetermined second transmission area, the predetermined first transmission area and the predetermined second transmission area being partially overlapping; and a mobile unit for maintaining communication among the mobile unit, the first stationary unit and the second stationary unit while the mobile unit is moving from the predetermined first transmission area to the predetermined second transmission area.
According to a fourth aspect of the current invention, a system for transmitting information via a common frequency carrier between a mobile unit and a set of stationary units, including: a first stationary unit for transmitting an information signal which is modulated by orthogonal frequency division multiplex modulation within a predetermined first transmission area; a second stationary unit for transmitting the information signal which is modulated by the orthogonal frequency division multiplex modulation within a predetermined second transmission area;, the predetermined first transmission area and the predetermined second transmission area being partially overlapping; and a mobile unit for maintaining communication among the mobile unit, the first stationary unit and the second stationary unit while the mobile unit is moving from the predetermined first transmission area to the predetermined transmission area.
These and various other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to the accompanying descriptive matter, in which there is illustrated and described a preferred embodiment of the invention.